Surface Preparation of Niobium - KEKepaper.kek.jp/srf80/papers/srf80-2.pdfIn the niobium matrix, suboxide clusters are inhomogeneously distributed in a layer of about 10 nm. A 0.5

SURFACE PREPARATION OF N I O B I U M

P. Kneisel

K e r n f o r s c h u n g s z e n t r u m K a r l s r u h e I n s t i t u t f u r K e r n p h y s i k P . B . 3 6 4 0 7500 K a r l s r u h e F e d e r a l R e p u b l i c o f Germany

1. Introduc t ion

Any d i s c u s s i o n of s u r f a c e p repa ra t i on f o r superconducting r f - s u r f a c e s i s ce r -

t a i n l y connected w i t h t h e ques t i on what i s t h e b e s t r e c i p e f o r ach iev ing h igh

Q-values and h igh break-down f i e l d s . Since t h e break-down i n a c a v i t y i s not

understood s o f a r and because s e v e r a l mechanisms p lay a r o l e , i t a l s o i s no t

pos s ib l e t o g i v e one r e c i p e which always works.

Nevertheless i n t h e p a s t c e r t a i n p repa ra t i on techniques f o r niobium s u r f a c e s

have been developed and c e r t a i n r u l e s f o r p r epa ra t i on can be a p p l i e d . I n t h e

fol lowing t h e to-days s t a t e of t h e a r t w i l l be desc r ibed . A d e s c r i p t i o n of t h e

physics of t h e s u r f a c e and t h e exp lana t ion of t h e phenomena observed i n r f - 1 , 2 c a v i t i e s caused by t h e s t a t e of t h e s u r f a c e w i l l be g iven a f t e rwards .

It i s we l l known t h a t r f - supe rconduc t iv i t y i s tak ing p l ace i n a t h i n s u r f a c e

l aye r of on ly a couple of nm. I n t h e ca se of niobium t h e e lec t romagnet ic f i e l d

p e n e t r a t e s about 60 nm i n t o t h e meta l depending on t h e mean f r e e path of t h e

e l e c t r o n s i n t h e s u r f a c e shea th .

It i s a l s o known t h a t from t h e po in t of view of r f - supe rconduc t iv i t y b e s t r e s u l t s

a s p r ed i c t ed by t h e BCS-theory can be achieved f o r a c l e a n , de fec t - f r ee meta l sur-

f a c e . I f t h i s s u r f a c e i s smooth on a microscopic s c a l e t h e s u r f a c e r e s i s t a n c e i s

smal le r than f o r a rough s u r f a c e because t h e c u r r e n t pa th s a r e s h o r t e r and there-

f o r e l e s s d i s s i p a t i o n takes p l a c e . A smooth s u r f a c e i s a l s o d e s i r a b l e , i f one is

i n t e r e s t e d i n e s t a b l i s h i n g high e lec t romagnet ic f i e l d s on the superconducting

s u r f a c e . A s i s known rough s u r f a c e s lead both t o e l e c t r i c f i e l d enhancements cau-

s ing non resonant e l e c t r o n loading , and t o magnetic f i e l d enhancements i n i t i a t i n g

rf-breakdown.

I n r e a l i t y of course one i s not d e a l i n g wi th i d e a l s u r f a c e s . The niobium i s i n

a d d i t i o n covered w i t h oxide adso rp t ion l a y e r s , t h e e f f e c t s of which w i l l be d i s -

cussed i n r e f . 2)

Proceedings of SRF Workshop 1980, Karlsruhe, Germany SRF80-2

In t h e fol lowing i t i s at tempted t o g ive a s h o r t d e s c r i p t i o n of t h e su r face

i n conjunct ion wi th the methods of su r face t r ea tmen t s , which g e n e r a l l y can

be app l i ed t o niobium c a v i t i e s .

2 . Surface Treatments

2.1 Machining of a Surface P.

For t h e shaping of metal p i eces d i f f e r e n t techniques a r e used a s w i l l be de-

s c r ibed i n d e t a i l l a t e r dur ing t h i s workshop. 3, Niobium,which i s e l e c t r o n

beam m o l t e n , i s u sua l ly e i t h e r machined on a l a t h e o r by m i l l i n g o r d i e formed

(hydroformed, spinned) by shee t metal techniques . I n a l l ca ses d e f e c t s a r e

introduced i n t o t h e m e t a l l i c su r face bes ide t h e d e f e c t s and impur i t i e s of t h e

s t a r t i n g m a t e r i a l .

0 Since niobium i s a l r e a d y a t low temperatures - 300 C ) r e a c t i n g wi th a i r 4 ) -

oxygen i s d isso lved i n then iob iummat r ix forming c l u s t e r s of suboxids which

enhance t h e v i c k e r s hardness - g r e a t c a r e has t o be taken t o avoid hea t ing of

t h e niobium p a r t dur ing machining. The choice of t h e l u b r i c a n t not only in-

f luences t h e temperature of t h e work-piece a t t h e t o o l , but a l s o the s u r f a c e

f i n i s h . Genera l ly , a l l l u b r i c a n t s conta in ing t r i ch lo r -e thy lene r e s u l t i n smooth

s u r f a c e s ; s u r f a c e roughnesses of 1-2 u m a r e obta ined . On a macroscopic s c a l e a

s e r i e s of h i l l s and v a l l e y s a r e p resen t , on a microscopic s c a l e smal le r i r r egu-

l a r i t i e s a r e superimposed a s ind ica t ed i n f i g . 1.

As important a s t h e roughness of t h e su r face i s the depth of t h e damage l aye r

introduced by t h e mechanical deformation of t he l a t t i c e dur ing machining. The

depth of t h i s su r face damage l aye r i s depending on t h e machining cond i t ion a s

w e l l a s on t h e c h o i c e o f t h e t o o l , i n f luenc ing e . g . t h e p res su re a t t h e t o o l ,

o r t he c u t t i n g a b i l i t y .

Pene t r a t ion depth measurements by Hauser 5, on niobium samples have ind ica t ed

t h a t t h e depth of t h e su r face damage l aye r f o r t he machining cond i t ions used

i n our l ab 6, i s i n the order of 50 Um. I f die-forming, hydro-famingor spinning

techniques a r e used, one would expect a t h inne r su r face damage l a y e r . Measurements

on c a v i t i e s a t Cornel l Un ive r s i ty , where shee t metal techniques have been developed

f o r t h e f a b r i c a t i o n of r e s o n a t o r s Y 7 ) i n d i c a t e t h a t a f t e r t he removal of about 50 pm

SRF80-2 Proceedings of SRF Workshop 1980, Karlsruhe, Germany

reasonable r e s u l t s a r e ob t a ined . For he l ix - r e sona to r s , which a r e b u i l d ou t of

drawn niobium - t ubes , t h e removal of a t l e a s t 100 pm was necessary .8) Ear ly

i n v e s t i g a t i o n s a t HEPL i n d i c a t e a s u r f a c e damage l a y e r of 250pm. 9 1

P r i o r t o t h e removal of t h i s s u r f a c e damage l a y e r a v i s u a l i n s p e c t i o n of t h e

s u r f a c e i s commonly done. Soaking t h e niobium i n water f o r a longer per iod o f

t ime i n d i c a t e s , i f t h e r e a r e i ron- inc lus ions i n t h e s u r f a c e , which show up a s

r u s t y spo t s and can i n d i v i d u a l l y be t r e a t e d . 10)

2 . 2 Chemical Sur face Treatment

For t h e removal of t h i s damage l a y e r e i t h e r e l e c t r o p o l i s h i n g o r chemical po l i -

shing i s commonly used. Both methods have g e n e r a l l y two e f f e c t s on a s u r f a c e :

a ) "smoothing" by e l i m i n a t i o n of l a r g e s c a l e i r r e g u l a r i t i e s ,

b ) "br ightening" by removal of superimposed smal le r i r r e g u l a r i t i e s .

E s s e n t i a l f o r t h e smoothing-effect du r ing e l e c t r o p o l i s h i n g i s the e x i s t e n c e of

a l a y e r of high v i s c o s i t y c o n s i s t i n g ou t of a s a t u r a t e d s o l u t i o n of r e a c t i o n

products . The r a t e of removal of metal ions from t h e s u r f a c e i s determined by

a d i f f u s i o n process i n t he v i scous layer ,which depends on t h e concen t r a t i on

g r a d i e n t s , t h e tempera ture , t h e a g i t a t i o n of t h e s o l u t i o n . 11)

A t p r o t r u s i o n s of t h e s u r f a c e , c u r r e n t d e n s i t i e s a r e high e s t a b l i s h i n g h igh

concen t r a t i on g r a d i e n t s , f a s t e r d i f f u s i o n of meta l i ons through t h e l aye r and

t h e r e f o r e a p r e f e r e n t i a l d i s s o l u t i o n of t h e peaks. A t v a l l e y s t h e c u r r e n t den-

s i t i e s a r e sma l l e r , a s a r e concen t r a t i on g r a d i e n t s ; and t h e r e f o r e d i f f u s i o n and

d i s s o l u t i o n a r e sma l l e r .

Chemical po l i sh ing of niobium i s u s u a l l y a s s o c i a t e d wi th a v igorous gas e v o l u t i o n ;

i n t h e ca se of niobium t h e evo lu t ion of t h e brownish n i t rogend iox ide t akes p l ace ;

i t has been suggested by s e v e r a l au tho r s 12 ) t h a t i n t h i s c a s e t h e v i s cous l a y e r

forms only i n t h e v a l l e y s and a t t h e peaks i t i s swept away by t h e t u r b u l e n t flow

of t h e s o l u t i o n due t o t h e gas evo lu t ion . Marked p r e f e r e n t i a l a t t a c k then occurs

a t t h e peaks l ead ing t o a r a p i d smoothing.

"Brightening" occurs only i f a t h i n s u r f a c e ox ide f i l m - on ly a few monolayers

t h i c k - i s cover ing t h e meta l t o be po l i shed . It prevents t he d i r e c t acces s


of t h e s o l u t i o n t o t h e s u r f a c e , which would cause p r e f e r e n t i a l d i s s o l u t i o n of

meta l from s i t e s of h igh energy r e s u l t i n g i n e t c h i n g (gra in-boundar ies , d i s -

l o c a t i o n s ) . It has been suggested t h a t t h e f i l m undergoes a cont inous process

of d i s s o l u t i o n by t h e ac id and renewal . I n o r d e r t o ma in t a in t h e f i l m , t h e

passage of meta l ions a c r o s s t h e m e t a l / f i l m i n t e r f a c e occurs a t t h e same r a t e

a t a l l p o i n t s . Such uniform a t t a c k of t h e meta l w i l l remove microscopic i r r egu -

l a r i t i e s from t h e s u r f a c e . 13)

A 1 though

f a c e s 11,

po l i sh ing

t h e r e e x i s t s e v e r a l methods of chemical t r ea tmen t of niobium sur -

14' 15' 16, t h e methods developed by Siemens company l') f o r e l e c t r o -

and by r e f . 9y18) f o r chemical p o l i s h i n g a r e most o f t e n used f o r

microwave c a v i t i e s . Table I shows t h e sununary of t h e app l i ed s u r f a c e t r e a t -

ments. Common t o a l l p roces ses i s t h e o x i d a t i o n of t h e niobium t o niobium - pentoxide , which i s d i s so lved i n excess h y d r o f l u o r i c a c i d a s o x i f l u o r i d e s .

The chemistry involved i s t h e fo l lowing: 19,20,21)

S t e p l +

e l e c t r o p o l i s h i n g : 2Nb + 5~0;- + 5H20 + Nb20s + 10H + 5~0, - + 10e

chemical p o l i s h i n g : 2Nb + 5N03 + NbzOs + 5N02 +

anod i z i ng : 2Nb + 5 0 ~ - + NbzOs + 5H + 10e

S t e p 2 Nb205+ 6HF + H2NbOF5+'NbO2F00.5H 0 '+1.5H20 s o l u b l e not s o l u b l e

S t e p 3 . Nb02F-0.5H20 + 4HF -, H2NbF5+ 1.5H20 s o l u b l e

The e l e c t r o p o l i s h e d o r chemica l ly pol i shed s u r f a c e s a r e contaminated wi th r e -

a c t i o n products , lower ox ides , s u l f u r and f l u o r i n e . 22323) Proper c l ean ing i s

achieved by r i n s i n g i n a d i l u t e d hydrogenperoxide s o l u t i o n du r ing u l t r a s o n i c

a g i t a t i o n and/or anodiz ing (ox ipo l i sh ing ) 10 '24y25) ; dur ing t h e s e s t e p s t h e

i n s o l u b l e reac t ion-products a r e conver ted i n t o s o l u b l e forms. I n t h e case of

t h e o x i p o l i s h i n g p roces s t h e r e a c t i v e niobium s u r f a c e i s i n a d d i t i o n s h i f t e d

towards "c leaner" r e g i o n s of t h e bulk m a t e r i a l . The amorphous Nb205 can t r a n s -

form i n t o a c r y s t a l l i n e mod i f i ca t ion , which appears a s "gray oxide" dur ing

numerous subsequent o x i p o l i s h i n g c y c l e s . A s Grundner 22) has found, t h e fo r -

mation of t h i s ox ide s t a r t s a t c e r t a i n n u c l e a t i o n s i t e s a s NbO, which qu ick ly

grow t o form a Nb 0 - l a y e r . This gray oxide i s not s o l u b l e i n Hf; i n most c a s e s 2 5

chemical methods a r e t o o weak, on ly mechanical methods a r e s u c c e s s f u l . The

p r o b a b i l i t y of t h e growth of t h i s c r y s t a l l i n e oxide i s reduced, i f t h e niobium


was hea t - t r ea t ed p r i o r t o ox ipo l i sh ing , which removes t h e s p o t s of h igh energy

(nuc l ea t i on s i t e s ) t o some extend. The s t a t e of t h e s u r f a c e - a s i n v e s t i g a t e d

by Grundner 2 2 ) wi th XPS - i s shown schemat ica l ly i n f i g . 2.

I n t h e niobium m a t r i x , suboxide c l u s t e r s a r e inhomogeneously d i s t r i b u t e d i n a

l a y e r of about 1 0 nm. A 0 . 5 nm t h i c k shea th of NbO and Nb20 i s sandwiched be t -

ween t h i s l a y e r and a Nb205 l a y e r , which i s contaminated by adso rba t e s .

2 .3 Heat Treatment

Some improvements of t h e s u r f a c e cond i t i on can be gained by a furnace t r e a t - 0

ment f o r temperatures above T 2 1600 C a s i nd i ca t ed i n t a b l e 11. 22) F i r s t of

a l l t h e niobium s u r f a c e i s cleaned from r e s i d u a l contaminat ion, 22 ) i f proper

r i n s i n g techniques a r e not a v a i l a b l e . 29) ~ u t even i n a ve ry good vacuum -8

(p I 1 0 t o r r ) t h e r e remain 1-2 atomic l a y e r s of s u r f a c e oxides (NbO, Nb20)

on t h e niobium, which a r e due t o oxygen seg rega t ion from t h e bu lk . For in-

c r ea s ing r e s i d u a l gas p re s su re s t h e ox ides grow a s a func t ion of temperature

and time. Typica l ly a Nb 0 - layer of 1 . 5 nm i s p re sen t on top of t h e niobium 2

a f t e r cool-down. Below 700 C t h e formation of suboxide c l u s t e r s i n t he niobium

ma t r ix which a r e due t o d i s s o l u t i o n of oxygen from t h e r e s i d u a l ga s , s t a r t s . 0

Therefore f a s t coo l ing below 700 C may be advantageous because of l e s s c l u s t e r i n g . 26)

Genera l ly t h e oxide l a y e r a f t e r a furnace t rea tment i s t h inne r than a f t e r chemical

t rea tment . There a r e i n d i c a t i o n s f o r l e s s e l e c t r o n loading of hea t - t r ea t ed sur-

f a c e s , i f p rov i s ions can be taken t o avoid adso rp t ion of H 0 , hydrocarbons dur ing 2

t h e handl ing of t h e c a v i t i e s a f t e r t h e furnace t rea tment ( e .g . glove box w i t h i n e r t

gas , f a s t assembly) . The main b e n e f i t s of a su r f ace t rea tment r a t h e r remain f o r

t h e bu1.k t han f o r t h e s u r f a c e :

0 0 a ) The niobium i s s t r e s s annealed and r e c r y s t a l l i z e d (900 C I T S 1200 C ) . In-

v e s t i g a t i o n s 27 ) a t Siemens company have shown t h a t work hardened m a t e r i a l

wi th a high d e n s i t y o f d i s l o c a t i o n s gave only moderate va lues of c r i t i c a l

magnetic f i e l d s ( ~ - b a n d - T M ~ ~ ~ - m o d e , H:' z 35 m ~ ) , whereas r e c r y s t a l l i z a t i o n

a t 1 2 0 0 ~ ~ improved t h e f i e l d s t o H:' 110-150 mT.

b ) The niobium i s homogenized (1000 I T 5 1300°c), which seems t o be ad-

vantageous f o r welded c a v i t i e s . I n t h i s temperature range t h e d i f f u s i o n

r a t e s 49) of impur i t i e s l i k e 0,C . , which have c l u s t e r e d i n t h e weld

during t h e welding process , a r e high enough t o r e s u l t i n a uniform


distribution. For heavily electropolished surfaces the dissolved hydrogen,

which clusters during cool-down to cryogenic temperatures and induces 0 stresses in the lattice, is removed during moderate firing (T 2 800 C).

c) Above 1 6 0 0 ~ ~ grains are growing due to secondary recrystallization. Al-

though investigations 27) comparing material of small grain size ( 1 mm)

and large grains ( > 10 mm) indicate no significant effect on the critical

magnetic field of a cavity, smoother surfaces are obtained during electro-

polishing for large grain material because of less grain boundary etching. 6

d) The thermal conductivity of the material is improved to some extend, if the 0 interstitial impurities are removed and the grains are growing (T 2 1800 C)

Better thermal conductivity is desirable for the improvement of break-down

fields in cavities. 28

3. Conclusion

There remain questions like:

- Is electropolishing to be prefered against chemical polishing? 0

- Is a high temperature firing (T > 1600 C) necessary or can it be avoided?

Let me conclude with a few statements:

Apparently the proper surface treatment has to be developed in each

laboratory and for each cavity-type. Also the material purity and

homogeneity is important. Nevertheless a few steps seem to be essential:

1) Trivially the surface damage layer has to be removed (50 pm - 250 urn).

Whether electropolishing or chemical polishing is the better method for

this purpose is not evident. At least it seems to be apparent that electro-

polishing results in a more uniform removal of material, whereas chemical

polishing tends to cause grain-boundary etching, if large amounts of material

have to be removed.

With both methods comparable results have been achieved. In X-band-cavities

peak electric field of E 60 to 70 M V / ~ corresponding to peak magnetic fields P

of H 110 mT have been obtained at H E P L ~~) and S L A C ~ ~ ) for chemically polished P

surfaces. At CORNELL 32) and at SIEMENS company 33'34) peak fields of E '55MV/m P

(H 150 mT) and H 110 - 150 mT, respectively, have been measured. At HEPL, 35) P

Wuppertal P 6 ) 36) and KfK bothmethods of chemical surface treatment have been


a p p l i e d w i t h comparable r e s u l t s a t S-band f r e q u e n c i e s .

A t 500 MHz i n t h e s t o r a g e r i n g c a v i t y of CERN 37) a c c e l e r a t i n g g r a d i e n t s of

Eacc 2 4.6 MV/m have been measured a f t e r a chemical p o l i s h i n g of t h e s u r f a c e .

The DORIS - t e s t c a v i t y a t KfK was o n l y t e s t e d w i t h e l e c t r o p o l i s h e d s u r f a c e s

and r e s u l t e d i n E - 4 .4 MV/m. 38

a c c

2 ) Proper r i n s i n g t e ch n iq ues a r e e s s e n t i a l t o remove s u r f a c e c on t am i na t i ons .

0 3 ) Heat t r e a tmen t a t moderate t empera tures ( T < 1300 C) s e r v e s ' t h r e e purposes:

a . Outgass ing of d i s s o l v e d hydrogen.

b . S t r e s s anne a l i n g , which seems t o be e s s e n t i a l f o r h e a v i l y co ld worked

m a t e r i a l and i s advantageous b e f o r e e l e c t r o p o l i s h i n g i n o r d e r t o avoid

p r e f e r e n t i a l e t c h i n g . For c a v i t i e s made o u t of s o l i d ( e l e c t r o n beam me l t e d )

m a t e r i a l a p p a r e n t l y n o t many s t r e s s e s a r e induced, 2 7 y 3 9 ) which makes a

h e a t t r e a tmen t unnecessa ry .

c . Homogenization, which seems i n most c a s e s t o be e s s e n t i a l f o r welded

c a v i t i e s , 2 9 y 32y40-43) a l t ho ugh t h e r e e x i s t e x c e p t i o n s . 36,371

0 4 ) High t em p e ra tu r e --- f i r i n g (T > 1600 C ) c a u s e s g r a i n growth, which does no t s i g -

n i f i c a n t l y i n f l u e n c e t h e performance of a c a v i t y . 27 ) I n a d d i t i o n a t h igh tem-

p e r a t u r e s and u l t r a h igh vacua t h e niobium i s p u r i f i e d due t o o u t ga s s ing of

d i s s o l v e d i n t e r s t i t i a l s , 44) bu t t h e r e seems t o be no e f f e c t on performance. 45)

Never the less h i g h t e m pe r a t u r e f i r i n g i s nece s sa ry t o a c h i e v e v e r y h i gh Q-values 46

(e.g,. Q > 10'' i n t h e TMOl0-mode a t X-band).

5 ) Dust f r e e assembly i s e s s e n t i a l bo th p r i o r t o f u rn ace t r e a tm e n t and t o r f - t e s t i n g .

Recen t ly t h e method of i o n s p u t t e r c l e a n i n g h a s been picked up a t CERN a s a method f o r

s u r f a c e c l e a n i n g of niobium c a v i t i e s . 47) About 2 y e a r s ago i n v e s t i g a t i o n s a t CORNELL

UNIVGRS ITY 48) w i t h i o n s p u t t e r c l e a n i n g have been s topped a s were t e s t s i n our labo-

r a t o r y 8 y e a r s ago because o f d i s c ou rag in g r e s u l t s . Hopeful ly t h e CERN - group can

demons t ra te t h e f e a s i b i l i t y o f t h i s method a s a f u r t h e r means t o improve supercon-

d u c t i n g r f - s u r f a c e s .



C on taminat ion

F i g . 2 : Schemat ic d i ag ram of a n iob ium sui - face

a s d e s c r i b e d by Grundner 22)


- 36 -

Table I : Chemical methods of su r face p repa ra t ion of niobium cav i t i e s

METHOD -

Elec tro-

polishing

Chemical-

polishing

Anodizing

(oxipolishing)

Rinsing

SOLUTION

H2S04(97%) :HF (40%

= 8 5 : l O

by volume

~~03(65%):~F (40%)

= 60 : 40

by volume

~NO3(65%):~F(40%):

~ 3 ~ 0 ~ ( 8 5 % ) = 1:l:l

20% NHeOH or

any diluted acid

except HF

Hz02 + dist. H20

+ ultrasonic

CONJIITIONS

10 - 15 V 2 5 - 35'~ current

oscillations

Room-temp. (RT)

or T<RT for

smaller reac-

tion rates

25 um/min at

oO C room-temp.

10 um/min

Room- t emp . 0 . . . 100 V voltage defines

thickness

- 2 . 4 nm/V

Room-temp.

15 - 30 min

with repeated

renewal

EFFECT

Smoothing

brightening

surface

roughness -.5 nm

Smoothing

(brightening)

grain boundary

edging

a) oxidation of

residual

oxides

b ) removal of S,F

c) neutralization

of acids

Very efficient

removal of sur-

face contamination

SURFACE

Contaminated

by reaction

product S

S, F . . .

Contamination

with reaction

product S

Sulfur removed

fluorine reduced

4


Ta

ble

11:

Ef

fe

ct

o

f

hig

h

tem

pe

ra

tur

e

fi

ri

ng

on

a

n

iob

ium

s

ur

fa

ce

----W

--

EF

FE

CT

M

ET

HO

D

Ul

tr

a

hig

h

va

cu

um

h

igh

te

mp

er

atu

re

f

ir

in

g

De

ga

ss

ing

o

f

ma

te

ri

al

----

----

----

----

----

----

----

----

C

ON

DIT

ION

S

16

00

-

19

00

~~

f

or

h

igh

B

-s

tru

ctu

re

s

I

12

oo

0c

s

ur

fa

ce

c

lea

nin

g:

F

rem

ov

ed

S

r

ed

uc

ed

C

co

nta

min

ati

on

d

ue

t

o r

in

si

ng

i

n

hy

dr

oc

ar

bo

ns

r

e-

d

uc

ed

t

o 1 m

on

o-

la

ye

r

SU

RF

AC

E

St

at

e

st

ro

ng

ly

d

ep

en

din

g

on

r

es

id

ua

l

ga

s

pr

es

su

re

i

n

fu

rn

ac

e :

A)

<1

0-'

T

or

r:

1-2

m

on

ola

ye

rs

o

f N

bO

, N

bzO

B P

inc

re

as

ed

: f

or

ma

tio

n

of

N

b2

O5

b

y

fu

rt

he

r

ox

ida

tio

n

as

f

un

cti

on

o

f

T,

t

?,

1.5

nm

fo

rm

ati

on

o

f s

ub

ox

ide

-c

lus

ter

s -

-----V

------

RE

MA

RK

S

e.g

. G

run

dn

er

+

du

e

to

s

eg

re

ga

ti

o

of

o

xy

ge

n

fro

m

bu

lk 1

+

re

sp

on

si

bl

e

fo

r

A)

st

ep

i

n A

UT

)

ar

ou

nd

7

K B

)

(Alk

Tc

)ex

p

< (

a/

k~

~)

~~

~

C)

R

# ~

(f

~)

F

as

t

co

ol-

do

wn

m

ay

re

du

ce

s

ub

ox

ide

c

lu

st

er

in

g.


R e f e r e n c e s --.-.--

A. Septier, "Surface Studies and Electron Emission", this workshop

J. Halbritter "Theoretical Aspects in RF-Superconductivity", this workshop

W. Bauer, "Fabrication of Niobium Cavities", this workshop

W. Schwarz, J. Halbritter, J. Appl. Phys. 48, p. 4618 (1977)

W. Hauser, KfK-Ext. Ber. 3/74-7, Kernforschungszentrum Karlsruhe (1974)

P. Kneisel, KfK-Bericht 1645, Kernforschungszentrum Karlsruhe (1972)

H. Padamsee, J. Kirchgessner,M. Tigner, R. Sundelin, M. Banner, J. Stimmell, L. Philips IEEE Trans. Magnetics, MAG-13, p. 346 (1977)

K.W. Zieher private comr.unication

J-.P. Turneaure, I. Weisman J. Appl. Phys. 38, p. 4417 (1968)

H. Padamsee, private communication

e.g. W.J. McTegart, The electrolytic and chemical polishing of metals, Pergamon Press (1959)

W.J. McTegart, R.G. Vines Trans. Aust. Inst. Metals 5, p. 107 (1952)

J. Edwards, J. Electrodep. Tech. Soc. 28, p. 133 (1952)

V. Sorajic, Metalloberflachen - Angewandte Elektrochem. 27, p. 80 (1973)

J. Votruba private communication

H. Lengeler private communic~tion

H. Diepers, 0. Schmidt, H. Martens, F.S. Sun Phys. Lett. 37A, p. 139 (1971)

M.L. Kinter, I. Weisman, W.W. Stein, J. Appl. Phys. 41, p. 828 (1970)

Gmelin Handbuch der Anorganiochen Chemie, Band 49 "NIOB"

D. Gunzel, L. List private communication

J. Pagetti, J. Talbot Corrosion-Traitements-Protection-Finition 15, p. 171 (1967)

e.g. M. Grundner KfK-Bericht 2565, Kernforschungszentrum Karlsruhe (1977) M. Grundner, J. Halbritter J. Appl. Phys. 51, p. 397 (1980)


G . Beranger , P. B o i s o t , P . Lacombe, G . Amsel, D . David Revue de Physique Appliqu6e 5 , p . 383 (1970)

H . Martens , H . D i epe r s , R . K . Sun Phys. L e t t . 34A, p . 439 (1971)

P. Kne i s e l , 0 . S t o l t z , J . H a l b r i t t e r , H . D i epe r s , H . Martens , R . K . Sun Proc . o f t h e 8 t h I n t e r n . Conf. on High Energy A c c e l e r a t o r s , p. 275 CERN, Geneva (1972)

P. Kne i s e l , 0 . S t o l t z , J . H a l b r i t t e r J . Appl. Phys. 45, p . 2296 (1974)

H . P f i s t e r e t a l . p r i v a t e communication

H . Padamsee "Heat Transfe r" , t h i s workshop

A. C i t r o n , G . Darmnertz, M . Grundner, L. Husson, R . Lehm, Nucl. I n s t r . and Methods 164, p . 31 (1979)

J . P . Turneaure , N.T. V i e t Appl .Phys. L e t t . 1 6 , p . 333 (1970)

P. B . Wilson, Z . D . F a rka s , H . A . Hogg, E.W. Hoyt, IEEE Trans . Nucl. S c i . NS-20, p . 104 (1973)

H . Padamsee, M . Banner, J . Ki rchgessner , M . T igne r , R . Sunde l in , IEEE Trans . Magnet ics . MAG-15, p . 602 (1979)

K . Schn i t zke , H . Martens , B . H i l l enb rand , H . Diepers Phys. L e t t . 45A, p . 241 (1973)

H . Martens , H. D i e p e r s , R . K . Sun Phys. L e t t . 44A, p . 213 (1973)

P. Kne i s e l , C . Lyne is , J . P . Turneaure , IEEE Trans . Nucl. S c i . NS-22, p . 1197 (1975)

U . K l e in , D . Proch, H . Lengeler Report WU B 80-16, Gesamthochschule Wuppertal (1980)

Ph. Bernard, G . C a v a l l e r i , E. C h i a v e r i , E. Haebel , H . He in r i c h s , H . Lengeler E. P i c a s s o , V . P i c c i a r e l l i , H . P i e l Pr'oc. of t h e 1 1 t h I n t e r n . Conf. on High Energy A c c e l e r a t o r s , CERN, Geneva (1980)

Sh. Noguchi, Y . Kojima, J . H a l b r i t t e r submi t ted t o Nucl. I n s t r . and Methods (1980)

P . Kne i s e l , H . Padamsee CLNS-Report 791433, Co rn e l l U n i v e r s i t y (1979)

G . Arno lds , H . H e i n r i c h s , W . Hoffmann, R . Mayer, N . M i n a t t i , H . P i e l , D . Proch, W . Weingar ten J . Appl. PHys. 47, p . 1134 (1976)

V . Lagomarsino, G.Manuzio, R . P a rod i 8 t h I n t e r n . Cryogenic Engeneering Co n f . , Genova, I t a l y (1980)

T. Furuya, K . Hosoyama, T. Kato, Y . Kojima, 0 . Konno Proceed ings of t h e 1979 L inea r Acc. Conf . , Brookhaven (1979)

J . P . Turneaure , H . A . Schwettman, H . D . Schwarz, M.S. McAshan Appl. Phys. L e t t . 25, p . 247 (1974)

M . S t r o n g i n , H . H . F a r r e l l , H . J . Halama, O.F. Kammerer, C . Varmazis, J . M . Dickey P a r t . Acc. 3 , p . 209 (1972)

H. P f i s t e r Cryogenics 16 , p . 17 (1976)


4 6 ) C.M. Lyneis, private communication

47) H . Lengeler private communication

48) H . Padamsee private communication

49) A . Joshi, M . Strongin Scripta ~etallurgica 8, p. '413 (1974)

A. Joshi, C. Varmazis, M. Strongin "Role of Carbon and Oxygen Interaction on Grain Growth and Purification of Niobium", BNL-Repor t , Brookhaven

50) H. Pfister et al. private corulnunication


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